FIG. 1 depicts a schematic diagram of a portion of a typical wireless telecommunications system, which provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals 101-1 through 101-3) that are situated within a geographic region. The heart of a typical wireless telecommunications system is Wireless Switching Center ("WSC") 120, which might be also known as a Mobile Switching Center ("MSC") or Mobile Telephone Switching Office ("MTSO"). Typically, Wireless Switching Center 120 is connected to a plurality of base stations (e.g., base stations 103-1 through 103-5) that are dispersed throughout the geographic area serviced by the system and to the local- and long-distance telephone offices (e.g., local-office 130, local-office 138 and toll-office 140). Wireless Switching Center 120 is responsible for, among other things, establishing and maintaining calls between wireless terminals and between a wireless terminal and a wireline terminal, which wireline terminal is connected to Wireless Switching Center 120 via the local and/or long-distance networks.
The geographic area serviced by a wireless telecommunications system is divided into spatially distinct areas called "cells." As depicted in FIG. 1, each cell is schematically represented by a hexagon; in practice, however, each cell has an irregular shape that depends on the topography of the terrain surrounding the cell. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with Wireless Switching Center 120.
For example, when wireless terminal 101-1 desires to communicate with wireless terminal 101-2, wireless terminal 101-1 transmits the desired information to base station 103-1, which relays the information to Wireless Switching Center 120. Upon receipt of the information, and with the knowledge that it is intended for wireless terminal 101-2, Wireless Switching Center 120 then returns the information back to base station 103-1, which relays the information, via radio, to wireless terminal 101-2.
Typically, the signal transmitted by a wireless terminal to a base station is radiated omni-directionally from the wireless terminal. Although some of the signal that is transmitted radiates in the direction of the base station and reaches the base station in a direct line-of-sight path, if one exists, most of the transmitted signal radiates in a direction other than towards the base station and is never received by the base station. Often, however, signals that radiate initially in a direction other than towards the base station strike an object and are reflected towards, and are received by, the base station. Thus, a signal can radiate from the wireless terminal and be received by the base station via multiple signal paths.
FIG. 2 depicts a schematic illustration of wireless terminal 101-1 as it transmits to base station 103-1. Signal 107-1 is received by base station 103-1 directly. Signal 107-2, signal 107-3, and signal 107-4 arrive at base station 103-1 after radiating initially in a direction other than towards base station 103-1 and only after reflecting off of an object, such as buildings 105-2 through 105-4, respectively. Signals 108-1 through 108-4 radiate from wireless terminal 101-1 but never reach base station 103-1.
Because each of the four signals arrives at base station 103-1 after having traveled a different path, each of the four signals arrives phase-shifted with respect to each other. The phase-shift of each signal is determined by the delay encountered by each signal in traversing its unique path. And furthermore, depending on the length of the path traveled and whether the signal is reflected off an object before reaching base station 103-1, the signal quality (e.g., the average power of an amplitude-modulated signal, the signal-to-noise ratio, absolute power in dBm, etc.) of each signal is different when received. This is partially due to the fact that when a signal is reflected off of an object, the degree to which the signal is attenuated is a function of, among other things, the angle at which the signal is incident to the object and the geometric and dielectric properties of the object.
In a code-division multiple access ("CDMA") wireless telecommunications system, each radio receiver endeavors to identify and isolate the highest-quality constituent signals incident on the receiver and to demodulate and combine them to estimate the transmitted signal. As is well-known in the prior art, this process is conducted with, among other things, a finger-assignor and a rake receiver. The finger-assignor repetitively scans the incoming composite signal, in well-known fashion, and attempts to identify the strongest constituent signals in the composite signal to the rake receiver. It is important to note that the scanning process may take a significant period of time to identify a new constituent signal appearing in the composite signal. This length of time is further aggravated by air-interface protocols (e.g., IS-95, etc.) that transmit in random bursts, because the constituent signal is not visible during scans that occur when the transmitted signal is gated off. The rake receiver isolates and demodulates each of the identified strongest constituent signals, and then combines the demodulation result from each constituent signal, in well-known fashion, to produce a better estimate of the transmitted signal than could be obtained from any single constituent signal. To accomplish this, a rake receiver comprises a plurality, but finite number, of individual receivers, known as "fingers," each of which isolates and demodulates one constituent signal.
As the wireless terminal moves, the relative signal quality and phase-shift of the constituent signals changes, sometimes considerably. Received constituent signals can disappear, new constituent signals can appear, and existing constituent signals can merge or diverge. The signal quality of a constituent signal can suffer radical momentary changes, which make it appear for a time that the constituent signal no longer exists, although it quickly reappears. Such changes can be due to, for example, Rayleigh fading, or the transmitter passing behind an obstruction. A finger can be mistakenly assigned to an apparent signal that is, in fact, noise, or a finger can be assigned to a genuine signal that disappears. These are called spurious signals, and, when assigned to the rake receiver: (1) degrade the quality of the demodulator's output, and (2) take up a finger in the receiver which could otherwise be profitably assigned to a genuine constituent signal.
FIG. 3 depicts a graph of the signal quality as a function of time of a constituent signal that has been assigned to a finger at t.sub.a.
A constituent signal can be de-assigned for various reasons. One reason a constituent signal might be de-assigned is that it is to be replaced with a constituent signal of apparently higher quality. Another reason is that the de-assigned constituent signal might have been determined to have become spurious. According to prior art, the constituent signal in FIG. 3 is de-assigned at time t.sub.d for this latter reason because its signal quality fell below the rejection threshold, R, for time t.sub.r.
A rake receiver will offer the best performance when its fingers are, at every instant, demodulating the best constituent signals, and not other signals. Therefore, the need exists for techniques that ensure that genuine constituent signals are found and utilized in the coherent combination process as quickly as possible, and that de-graded and spurious signals are removed as quickly as possible.